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<h3><a name="SECTION0016325000000000000000"></a><a name="optparse-default-values"></a>
<br>
14.3.2.5 Default values
</h3>

<p>
All of the above examples involve setting some variable (the
``destination'') when certain command-line options are seen.  What happens
if those options are never seen?  Since we didn't supply any defaults,
they are all set to <code>None</code>.  This is usually fine, but sometimes you
want more control.  <tt class="module">optparse</tt> lets you supply a default value for each
destination, which is assigned before the command line is parsed.

<p>
First, consider the verbose/quiet example.  If we want <tt class="module">optparse</tt> to set
<code>verbose</code> to <code>True</code> unless <code>"-q"</code> is seen, then we can do this:
<div class="verbatim"><pre>
parser.add_option("-v", action="store_true", dest="verbose", default=True)
parser.add_option("-q", action="store_false", dest="verbose")
</pre></div>

<p>
Since default values apply to the <em>destination</em> rather than to any
particular option, and these two options happen to have the same
destination, this is exactly equivalent:
<div class="verbatim"><pre>
parser.add_option("-v", action="store_true", dest="verbose")
parser.add_option("-q", action="store_false", dest="verbose", default=True)
</pre></div>

<p>
Consider this:
<div class="verbatim"><pre>
parser.add_option("-v", action="store_true", dest="verbose", default=False)
parser.add_option("-q", action="store_false", dest="verbose", default=True)
</pre></div>

<p>
Again, the default value for <code>verbose</code> will be <code>True</code>: the last
default value supplied for any particular destination is the one that
counts.

<p>
A clearer way to specify default values is the <tt class="method">set_defaults()</tt>
method of OptionParser, which you can call at any time before calling
<tt class="method">parse_args()</tt>:
<div class="verbatim"><pre>
parser.set_defaults(verbose=True)
parser.add_option(...)
(options, args) = parser.parse_args()
</pre></div>

<p>
As before, the last value specified for a given option destination is
the one that counts.  For clarity, try to use one method or the other of
setting default values, not both.

<p>

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<span class="release-info">Release 2.5.1, documentation updated on 18th April, 2007.</span>
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